Hoist systems in the prior art are used in the offshore industry in the form of drilling derricks on, for example, drilling vessels. When, in use, a drill string is attached to the bottom side of a moveable, also known as a traveling block. The moveable trolley runs on separate tracks inside or outside the derrick.
The problem in the current art is that it is difficult to find an optimum compromise between speed and power in the hoisting systems. The hoisting cable is guided over the cable blocks in the derrick in such a way that several cable parts extend between the derrick and the moveable block. In this case, the more the wire parts are present between the derrick and the moveable block, the greater the load that can be lifted with the hoist system if the hoisting winch remains unchanged. In contrast, the more wire parts present between the derrick and the moveable, the lower the speed at which the moveable can be moved relative to the derrick when the maximal speed of the winch stays the same.
In order to find a good compromise between speed and lifting power, it is generally decided to provide the hoist system with relatively heavy winches. The heavy winches ensure that they will be able to move the move up and down rapidly in every conceivable case. The disadvantage is that a substantial part of the lifting power is not being utilized for a substantial part of the time. In other words, the device is actually provided with a winch that is too heavy—and therefore too expensive—to be able to reach sufficient speed occasionally.
The object of the current invention is to provide a hoisting system that, on one hand, can handle a relatively heavy load and, on the other hand, handle a relatively light load at a relatively high speed. Such a design could be relatively light and cheap design.
The object is achieved in the present invention by guiding the hoisting cable over loose pulleys that can be moved between a first position, in which the loose pulleys are connected to the derrick, and a second position, in which the loose pulleys are connected to the moveable.
The effect of this measure is that the number of wire parts between the derrick and the moveable can be set to optimum number. When loose pulleys are attached to the derrick, few wire parts extend between the derrick and the moveable and, therefore, a relatively low weight can be lifted with a relatively high speed. When loose pulleys are attached to the moveable trolley, a relatively large number of wire parts extend between the derrick and the moveable and, therefore, the moveable trolley can be moved at a relatively low speed relative to the derrick with a relatively large load. Since the hoisting cable is guided over the pulleys, the pulleys can be attached as desired to the derrick or to the moveable. The hoisting cable does not have to be reeved again and the desired number of wire parts can be set in a relatively short time.
According to the current invention, the loose pulleys can be attached symmetrically relative to the center of the derrick. This symmetry ensures that the forces exerted upon the cables are transmitted symmetrically to a derrick. In turn, no additional bending loads are exerted upon the derrick to limit the necessary weight of the derrick.
The loose pulley can also be placed in a housing with locking elements for fixing the pulleys on the moveable. The loose pulleys are pulled automatically into their first position, in contact with the derrick, by tension in the hoisting cable. It is therefore sufficient to provide the bottom side of the pulleys with locking elements.
It is advantageous for the locking elements to be equipped with a hydraulic actuation device. The use of a hydraulic actuation device means that the locking pins can be remotely controlled.
In the devices according to the prior art it is customary for a hoisting cable to be attached to a fixed point at one end. The other end of the hoisting cable is then wound around a hoist winch. If this hoist winch breaks down, it is no longer possible to work with the device. The mentioned hoist winch has also to be of relatively large and costly design to meet with all the required demands. Repeated bending at the same places is a major factor of wear of the cable. To increase the service life of the cable after a known number of lifting cycles the cable is shifted to move the places of repeated bending. In hoist systems known from prior art this is done by a procedure known as the “slip & cut” procedure. This takes considerable time and is not without personal danger.
It is therefore an object of this invention to provide a hoist system by means of which an increased level of redundancy is provided. It is another object of this invention to provide means with which the time consuming and dangerous “slip & cut” procedure can be avoided altogether. An object of this invention is to provide a hoist system with relatively inexpensive winches decreasing the building and operating cost of the hoist system.
It is therefore advantageous for the hoisting means to be provided with two winches, each end of the hoisting cable being wound onto a separate winch. By winding the two ends onto a separate winch, it is possible to achieve the same cable speed at a relatively low speed of revolution of the winches. By using two winches the cable can be shifted automatically a distance from one winch to the other winch replacing the “slip & cut” procedure. This takes considerably less time and can be performed completely automatic reducing the chance of personal injuries.
Moreover, by adding the second winch, redundancy is provided in the system. Should one of the winches fail, then the hoist system is not unusable, but it is possible to continue working with a single winch.
It is advantageous for the winches to be driven by a plurality of relatively small motors. Because of the fact that twice as many sides of the winches can be used to attach the motor on these motors can be relatively small. For example, it is possible to equip the winches on both sides with electric motors that engage with a pinion in a toothed wheel of the winch. First, this has the advantage that such electric motors are commercially available. For the use of the hoist system it is therefore not necessary to develop a special, and therefore expensive, hoisting winch. Secondly, the relatively small motors have a low internal inertia, which means, for example, that when the direction of rotation of the winch is reversed less energy and time are lost during the reversal.